Fundamentals of MBR design
The design of a membrane bioreactor system largely relates to:
- the configuration of the membrane,
- the membrane separation process, and
- the biotreatment process.
1. Membrane separation process configuration:
Membrane configuration concerns the geometry of the membrane (hollow fibre, flat sheet or tubular) and the direction in which the water flows through it (in-to-out or out-to-in).
The membrane separation process configuration concerns the placement of the membrane module in the overall MBR process, i.e. either inside or outside the tank.
2. Biotreatment process configuration:
The biotreatment process configuration determines the biochemistry and, therefore:
- which pollutants are removed (organic carbonaceous materials, ammoniacal compounds and nutrients), and
- which products are formed (carbon dioxide or methane, nitrate or nitrogen, etc).
Biotreatment using air is referred to as ‘aerobic treatment’, leading to carbon dioxide as the main carbon-based product. In the absence of air, biotreatment is termed ‘anaerobic treatment’, which yields methane.
Biotreatment processes can be configured either as ‘fixed film’ or ‘suspended growth’. An MBR is a suspended growth process, based on the classical activated sludge (CAS) process but with membrane separation rather than sedimentation of the retained biological solids.
Membrane technology is integrated with a biological process either as a filter for retaining the biomass, as in an MBR, or as a diffuser for introducing air or oxygen in the molecular or bubbleless form, as in a membrane aerated biofilm reactor or MABR.
Membrane filtration may also be used downstream of a CAS for removing residual solids. In this case it is not integrated with the biological process and may be referred to as a 'polishing' step.
Further elements of design comprise pretreatment (screening and, for some applications, clarification) and post-treatment (generally either disinfection or desalination).
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